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(Abridged) Among the various issues that remain open in the field ofaccretion onto black hole X-ray binaries (BHBs) is the way the gas accretes atvery low Eddington ratios, in the so-called quiescent regime. While there isgeneral agreement that the X-rays are produced by a population of high-energyelectrons near to the BH, the controversy comes about in modeling thecontribution from inflowing vs. outflowing particles, and their relative energybudget. Recent Spitzer observations of three quiescent BHBs have shown evidencefor excess emission with respect to the tail of the companion star between 8-24micron. We suggest that synchrotron emission from a partially self-absorbedoutflow might be responsible for the observed mid-IR excess, in place of, or inaddition to, thermal emission from circumbinary material. If so, then the jetsynchrotron luminosity exceeds the measured 2-10 keV luminosity by a factor ofa few in these systems. In turn, the mechanical power stored in the jet exceedsthe bolometric X-ray luminosity at least by 4 orders of magnitude. We thencompile the broadband spectral energy distribution (SED) of A0620-00, thelowest Eddington-ratio stellar mass BH with a known radio counterpart, by meansof simultaneous radio, optical and X-ray observations, and the archival Spitzerdata. We are able to fit the SED of A0620-00 with a `maximally jet-dominated'model in which the radio through the soft X-rays are dominated by synchrotronemission, while the hard X-rays are dominated by inverse Compton at the jetbase. The fitted parameters land in a range of values that is reminiscent ofthe Galactic Center super-massive BH Sgr A*. Most notably, the inferred ratioof the jet acceleration rate to local cooling rates is two orders of magnitudeweaker with respect to higher luminosity, hard state sources.

The spectral energy distribution of quiescent black hole X-ray binaries: new constraints from Spitzer